1. Technical Field of the Invention
The present invention relates to a channel-selection-type demultiplexing circuit provided in an ADM device (Add-Drop Multiplexer) of a communication system, for demultiplexing ultra-high-speed multi-channel multiplexed signal streams.
2. Background Art
As the telephone-based communication networks of the past change into multi-media communication networks transmitting multi-media traffic such as computer information, high-capacity trunk transmission networks which can handle gigabit traffic will be required. Additionally, the conditions demanded of trunk transmission networks are predicted to diversify to cope with multi-media traffic. For constructing these high-capacity trunk networks, an ultra-high-speed ADM device is a key network element.
ADM devices are devices used when an N-channel multiplexed signal streams is sent from a transmitting station to a local station, for dropping the signals of specified channels at the local station, adding other signals to the same channels which had been assigned the dropped signals, and sending these along with the signals which were not dropped to a third station.
FIG. 11 is a diagram showing an example of the structure of a demultiplexing section SC11 in a conventional ADM device.
The demultiplexing section SC11 of this conventional ADM device first demultiplexes an N-channel multiplexed signal stream data-in at the demultiplexing section DMUX, then performs TSI (Time-Slot Interchange) of the signals of these demultiplexed channels using a channel selector having an ACM (Address Control Memory), after which add/drop selection is performed by an add/drop selector.
The channel selector used in the above conventional example is a type of memory. When N-channel multiplexed signal streams data-in are to be stored in this memory, each channel number and output port to be output from the memory are addressed for each N-channel multiplexed signal stream.
As conditions required of ADM devices in multi-media communication networks, the multiplexing speeds must become faster, the signal speeds of slower channels (tributary channels) must become faster, and the signal speeds of channels which are transmitted without being dropped (through-channels) must also become faster in order to cope with increased capacity. Furthermore, the signal speeds of channels handled by ADM devices will need to diversify for multi-media communications.
In order to put a conventional ultra-high-speed ADM device into practice, TSI of the ultra-high-speed multi-channel multiplexed signal streams must be performed. However, the TSI of multiple channels requires large-scale integrated circuits proportional to the number of channels. Additionally, if the speed of the transmission path which is the unit of TSI is increased, then a high-speed-operation memory in proportion to the speed of the path is also necessary. When the memory cannot operate at high speeds, the transmission path must be expanded until a speed at which the memory is capable of operating. However, in order to perform this expansion, the integrated circuits must be made larger still. Furthermore, when the speed of the transmission path which is the unit of TSI is diversified, then there are problems in the scale of circuitry with structures which use memories that operate at constant speeds.